These plants had looked really good when I bought them, but ever since, their leaves have been turning brown and crunchy. I did end up letting their soil completely dry on accident and I think that's what killed most of the Soleirolia. I'm assuming they're dying because they're not getting enough humidity. Will they eventually grow back if I give them more humidity? Will the roots left in the soil grow new stems and leaves?? Also, could I propagate the leaves of the Pilea that had fallen off that are still alive?

I believe you have Pilea depressa and P. libanensis but the resolution of the first is too low for me to see if the leaf edge has a slight scallop or tooth. In any case I think both can recover if the are given strong indirect light (or maybe a touch of morning sun), and great care not to overwater. Humidity is probably less important, unless you are running an air conditioner.
Pilea do not propagate from single leaves.